A soil has pH 9. What is the general idea to balance it for crops?

Correct answer: Carefully add organic matter that gives an acidic effect

A pH of 9 indicates that the soil is distinctly basic or alkaline. To move its pH toward a range more suitable for many crops, the general principle is to add a suitable mildly acidic amendment, often organic matter such as compost or well-managed manure, according to soil testing. Therefore option A is the best general answer. Slaked lime and sodium hydroxide would increase alkalinity, while soap is not an appropriate soil amendment.